1. Field of the Invention
The present invention relates to a photoelectric converting semiconductor device and, more specifically, to a photoelectric converting semiconductor device in which impedance variation is suppressed when the photoelectric converting semiconductor element is mounted on a board.
2. Description of the Background Art
A conventional photoelectric converting semiconductor device will be described. As can be seen from FIG. 16, a photoelectric converting semiconductor device includes a photoelectric converting semiconductor element 101, a coplanar waveguide board 102 (hereinafter referred to as xe2x80x9ccoplanar boardxe2x80x9d) for feeding a modulation signal voltage (hereinafter referred to as xe2x80x9cRF signalxe2x80x9d) to photoelectric converting semiconductor element 101, a terminal resistance 103 for impedance matching and a coupling optical system for input/output (not shown).
On coplanar board 102, a signal line 104 and a ground line 105 are formed. Photoelectric converting semiconductor element 101 is electrically connected to signal line 104 through a bump electrode 106a, and electrically connected to ground line 105 through bump electrodes 106b to 106d. Signal line 104 and ground line 105 are electrically connected through a terminal resistance 103.
An operation of the photoelectric converting semiconductor device will be described. When the photoelectric converting semiconductor element 101 is an electric field absorbing type semiconductor optical modulator element (hereinafter referred to as xe2x80x9coptical modulator elementxe2x80x9d), a continuous laser beam is introduced with high efficiency from the incident side coupling optical system to the optical modulator element.
In the optical modulator element, the amount of laser beam absorption changes in accordance with the voltage applied through coplanar board 102. Therefore, by applying a modulation signal voltage to coplanar board 102, the laser beam emitted from the optical modulator element has its intensity modulated corresponding to the signal voltage, and thus, it is coupled with high efficiency to the emitting side coupling optical system. The conventional photoelectric converting semiconductor device is structured and operates in this manner.
The conventional photoelectric converting semiconductor device, however, has the following problem. First, in the photoelectric converting semiconductor device, in order to have the impedance of the RF signal feeding side (feeding side impedance) matched with characteristic impedance of coplanar board 102, the width of signal line 104 and the distance between signal line 104 and ground line 105 on coplanar board 102 are set to a prescribed width and a prescribed distance.
Here, the width of the signal line 104 and the distance between signal line 104 and the ground line 105 are designed such that the characteristic impedance matches the feeding side impedance with the coplanar board 102 being in a single body state, that is, when photoelectric converting semiconductor element 101 is not yet mounted on coplanar board 102.
Therefore, when photoelectric converting semiconductor element 101 is mounted on coplanar board 102, the characteristic impedance tends to deviate from the value of the feeding side impedance, degrading electrical characteristic of the photoelectric converting semiconductor device.
The present invention was to made to solve the above described problem, and its object is to provide a photoelectric converting semiconductor device of which variation of characteristic impedance is suppressed when the photoelectric converting semiconductor element is mounted on a coplanar board.
According to the present invention, the photoelectric converting semiconductor device has a board, a signal line, a ground line, a resistance portion and a photoelectric converting semiconductor element. The signal line is formed on and extends over the board. The ground line is formed on the board and extends spaced apart from the signal line. The resistance portion is formed on the board and electrically connects the signal line and the ground line. The photoelectric converting semiconductor element is mounted on the board to cover the signal line and the ground line, electrically connected to the signal line and the ground line to receive a modulation signal transmitted from a power feeding portion for transmitting the modulation signal, and modulates and outputs the received light. The impedance is substantially the same as the impedance of the power feed portion. In order to suppress impedance variation when the photoelectric converting semiconductor element is mounted on the board, an arrangement relation between the signal line and the ground line positioned in an area where the photoelectric converting semiconductor element is mounted is made different from the arrangement relation between the signal line and the ground line positioned in an area where the photoelectric converting semiconductor element is not mounted.
According to this structure, the arrangement relation between the signal line and the ground line positioned in the area where the photoelectric converting semiconductor element is mounted is made different from the arrangement relation between the signal line and the ground line positioned in an area where the photoelectric converting semiconductor element is not mounted, and therefore, variation of the impedance is suppressed when the photoelectric converting semiconductor element is mounted on the board, and the impedance can be set to a value substantially the same as the impedance of the power feed portion. As a result, degradation of electric characteristic of the photoelectric converting semiconductor device can be prevented.
More specifically, the signal line and the ground line positioned in an area where the photoelectric converting element is not mounted extend spaced by a first distance from each other, and the signal line and the ground line positioned in an area where the photoelectric converting semiconductor element is mounted extend spaced by a second distance, which is wider than the first distance.
When the photoelectric converting semiconductor element is brought close to the board, correlation between the characteristic impedance and the distance between the signal line and the ground line shifts from that of the board alone. Here, when the distance between the signal line and the ground line positioned in the area where the photoelectric converting semiconductor element is mounted is made to a second distance wider than the first distance, characteristic impedance variation when the photoelectric converting semiconductor element is mounted on the board can be suppressed, and the value of the characteristic impedance can be set to substantially the same value as the feeding side impedance.
More specifically, the signal line positioned in the area where the photoelectric converting semiconductor element is mounted should preferably have a prescribed width narrower than the width of the signal line positioned in the area where the photoelectric converting semiconductor element is not mounted.
Thus, in the area where the photoelectric converting semiconductor element is mounted, the distance between the signal line and the ground line is substantially made wider than the distance in the area where the photoelectric converting semiconductor element is not mounted. Thus, characteristic impedance variation when the photoelectric converting semiconductor element is mounted on the board can be suppressed, and the value of the characteristic impedance can be set to a value substantially the same as the feeding side impedance.
More specifically, the semiconductor device further includes an additional ground line positioned along the direction of extension of the signal line on the side opposite to the ground line and electrically connected to the ground line, and the distance between the ground line and the additional ground line positioned in the area where the photoelectric converting semiconductor element is mounted is wider than the distance between the ground line and the additional ground line positioned in the area where the photoelectric converting semiconductor element is not mounted.
In this case also, the distance between the signal line and the ground line in the area where the photoelectric converting semiconductor element is mounted is substantially wider than the distance in the area where the photoelectric converting semiconductor element is not mounted. Thus, characteristic impedance variation when the photoelectric converting semiconductor element is mounted on the board can be suppressed, and the value of the characteristic impedance can be set to a value substantially the same as the feeding side impedance.
Further, the semiconductor device includes an additional ground line arranged extending along the direction of extension of the signal line on the other side of the ground line and electrically connected to the ground line, and when the photoelectric converting semiconductor element has an electrode portion electrically connecting the ground line and the additional ground line, it is preferred that the width of the signal line is narrower than the prescribed width.
Thus, the variation of the characteristic impedance derived from parasitic capacitance between the signal line and the electrode portion can be suppressed, and the value of the characteristic impedance can be set to a value substantially the same as the feeding side impedance.
Further, it is preferred that the width of the signal line changes abruptly from the area where the photoelectric converting semiconductor element is mounted to the area where the photoelectric converting semiconductor element is not mounted, and that the photoelectric converting semiconductor element is mounted aligned with the portion where the width changes abruptly.
Thus, the photoelectric converting semiconductor element is mounted on the board aligned with that portion at which the width of the signal line changes. As a result, the position for mounting is made clear, improving accuracy of mounting. As a result, the characteristic impedance variation caused as the mounting position of the photoelectric converting semiconductor element varies can be suppressed, and degradation of electric characteristic of photoelectric converting semiconductor device can be prevented.
Alternatively, the width of the signal line is preferably changed smooth from the area where the photoelectric converting semiconductor element is mounted to the area where the photoelectric converting semiconductor element is not mounted.
Here, reflection on the signal line is suppressed, and troubles resulting from the reflection can be eliminated, so that degradation of the electric characteristic of the photoelectric converting semiconductor device can be prevented.
Further, it is preferred that the signal line and the ground lines positioned in the area where the photoelectric converting semiconductor element is not mounted are arranged on one same plane, while the signal line and the ground lines positioned in the area where the photoelectric converting semiconductor element is mounted are arranged on mutually different planes.
In this case also, characteristic impedance variation when the photoelectric converting semiconductor element is mounted on the board can be suppressed significantly, and in the state where the photoelectric converting semiconductor element is mounted on the board, the value of the characteristic impedance is made substantially the same as remaining two impedance values, whereby degradation of the electric characteristic of the photoelectric converting semiconductor device can be suppressed.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.